From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DD77BEB64DA for ; Wed, 19 Jul 2023 11:21:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229679AbjGSLVn (ORCPT ); Wed, 19 Jul 2023 07:21:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48046 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229613AbjGSLVl (ORCPT ); Wed, 19 Jul 2023 07:21:41 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [IPv6:2001:67c:2178:6::1d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5BF59189 for ; Wed, 19 Jul 2023 04:21:40 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 1D0B71FE99; Wed, 19 Jul 2023 11:21:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1689765699; h=from:from:reply-to:reply-to:date:date:message-id:message-id:to:to: cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=CL6zWa7lw7sIuV2Jg643OeYAj2G6p2CpNAc2SRUEpQ8=; b=1CeUjh5s3vg1cFto/9ZDbGCTVdszTdb/M+tyhGlUQxwvsrB8AHvVrFT7t5WkeyxY0hQwDH 1t/iQvOl8+shHCgyHbeepkTM2rThTHED33laTdjpendMjC6DxEp+xbkf41s6Y6Q6Nh2Mkw Dd7sOfIysAdyMYEDZ0iiP/Nz4l7wUI8= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1689765699; h=from:from:reply-to:reply-to:date:date:message-id:message-id:to:to: cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=CL6zWa7lw7sIuV2Jg643OeYAj2G6p2CpNAc2SRUEpQ8=; b=rTA7yFELXf0R91FljTJmH6Wj2AJV4CnHYHQSVTpuCNovMynOp3/G7Zc75oY9bS3RdzAJMa qwhG8RM+wOWSBKCA== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 51B331361C; Wed, 19 Jul 2023 11:21:38 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id 2mJvEkLHt2S8XQAAMHmgww (envelope-from ); Wed, 19 Jul 2023 11:21:38 +0000 Date: Wed, 19 Jul 2023 13:21:36 +0200 From: Petr Vorel To: Cyril Hrubis Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Valentin Schneider , linux-kernel@vger.kernel.org, Jiri Bohac , ltp@lists.linux.it, Shile Zhang , Shile Zhang Subject: Re: [LTP] [PATCH 1/2] sched/rt: Fix sysctl_sched_rr_timeslice intial value Message-ID: <20230719112136.GA1239020@pevik> Reply-To: Petr Vorel References: <20230719103743.4775-1-chrubis@suse.cz> <20230719103743.4775-2-chrubis@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230719103743.4775-2-chrubis@suse.cz> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, [ Cc Shile Zhang ] > Thre is 10% rounding error in the intial value of the > sysctl_sched_rr_timeslice with CONFIG_HZ_300=y. > This was found with LTP test sched_rr_get_interval01: > sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed > sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns > sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90 > sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed > sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns > sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90 > What this test does is to compare the return value from the > sched_rr_get_interval() and the sched_rr_timeslice_ms sysctl file and > fails if they do not match. > The prolem it found is the intial sysctl file value which was computed as: > static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE; > which works fine as long as MSEC_PER_SEC is multiple of HZ, however it > introduces 10% rounding error for CONFIG_HZ_300: > (MSEC_PER_SEC / HZ) * (100 * HZ / 1000) > (1000 / 300) * (100 * 300 / 1000) > 3 * 30 = 90 > This can be easily fixed by reversing the order of the multiplication > and division. After this fix we get: > (MSEC_PER_SEC * (100 * HZ / 1000)) / HZ > (1000 * (100 * 300 / 1000)) / 300 > (1000 * 30) / 300 = 100 > Signed-off-by: Cyril Hrubis > CC: Jiri Bohac > --- > kernel/sched/rt.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index 00e0e5074115..185d3d749f6b 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -25,7 +25,7 @@ unsigned int sysctl_sched_rt_period = 1000000; > int sysctl_sched_rt_runtime = 950000; > #ifdef CONFIG_SYSCTL > -static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE; > +static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC * RR_TIMESLICE) / HZ; It looks like very old bug, from v4.11-rc1. I guess this should go to all stable and LTS kernels. Fixes: 975e155ed873 ("sched/rt: Show the 'sched_rr_timeslice' SCHED_RR timeslice tuning knob in milliseconds") Reviewed-by: Petr Vorel Kind regards, Petr > static int sched_rt_handler(struct ctl_table *table, int write, void *buffer, > size_t *lenp, loff_t *ppos); > static int sched_rr_handler(struct ctl_table *table, int write, void *buffer,